For over a century, the Hell Creek and Fort Union formations and their constituent fossil biotas have captivated geologists and paleontologists alike. In Montana and adjacent areas, these rocks have become renowned as the type locality for Tyrannosaurus rex and the epicenter for debate surrounding the mass extinction of dinosaurs at the Cretaceous-Paleogene boundary. The chapters in this volume represent a surge of field and laboratory research activity that illustrates the impacts of new and refined methods and tools. In tandem, the research questions have evolved to take advantage of the increased precision, quality, and quantity of the data, from determinations of paleoecologies to assessment of ontogenetic sequences, patterns of sedimentation, and basin-level intraformational correlations. Together, the chapters in this volume are a major step forward in the quest to mine the rich lode of geologic and biologic history preserved in the strata bounding the Cretaceous-Paleogene boundary.